The present embodiments relate to a shim coil arrangement for at least one extremity of a patient (e.g., a forearm and/or a hand) for use in a magnetic resonance device.
Magnetic resonance imaging and magnetic resonance devices suitable for magnetic resonance imaging are known in the prior art. Imaging in magnetic resonance tomography is based on the spins of atomic nuclei that are aligned in a main magnetic field (B0). Also known in the prior art are local coils that are used when the best possible signal-to-noise ratio (SNR) is to be achieved. Local coils are antenna systems that are arranged in immediate proximity to (e.g., on (anterior) or under (posterior)) the patient. During the acquisition of magnetic resonance image data, the excited nuclei induce a voltage in the individual antennas or coil elements of the local coil. The induced voltage is preamplified by a low-noise preamplifier (LNA) and forwarded to a receiving device that processes the magnetic resonance image data further. Efforts to achieve a further improvement in the signal-to-noise ratio (e.g., in the case of high-resolution magnetic resonance images) are centered on the use of systems referred to as high-field systems having a basic magnetic field that has a strength of 1.5 T to 12 T or more.
The homogeneity of the basic magnetic field, the B0 field, of the magnetic resonance device is important for all magnetic resonance imaging purposes. Excessively strong deviations from the homogeneity in the imaging volume may result in artifacts or distortions. Certain applications such as fat saturation, for example, may not be executed when a specific inhomogeneity of the basic magnetic field is present.
Fat saturation (e.g., fatsat) is an imaging technique in which the frequency shift of the protons bound in fat or fat-like materials is used in order to “mask out” the signals originating from the fat tissue by a saturation pulse as a transmit pulse at the fat frequency. Since the difference between the frequency of protons in water and the frequency of protons in fat is very slight (e.g., equivalent to a few ppm of the basic magnetic field), the imaging technique is heavily dependent on the spatial homogeneity of the basic magnetic field.
Efficiently functioning fat saturation is important for many diagnostic problems. This is because pathological tissue exhibits similar or the same contrast behavior as fat in many sequence types. Known fat saturation methods are, for example, the Dixon method and the spectral fat saturation technique.
In spectral fat saturation and related techniques, the fact that protons in fat and protons in water have slightly different resonant frequencies is used. The difference in frequency between protons in fat and protons in water may amount, for example, to approximately 3.1 ppm. A strong transmit pulse on the fat frequency suppresses the signal of the fat protons without affecting the imaging of the protons belonging to the water molecules. The functional effectiveness of all techniques based on the spectral separation of fat and water is dependent on the homogeneity of the basic field. If the basic field exhibits inhomogeneities of a similar order of magnitude to the spectral separation of protons in fat and water, then the fat and water resonances lie on the same resonant frequency and may no longer be separated spectrally.
With known magnetic resonance devices, magnetic field homogeneities having deviations of less than 1 ppm over a volume of approximately 30×40×50 cm may be attained. Problems with inhomogeneities may therefore arise, for example, in regions of the anatomy that lie at the extremities, (e.g., in the region of the shoulder) which may not be positioned centrally due to the lack of space in the patient receiving bore of a magnetic resonance device.
Inhomogeneities arising due to the magnetic resonance devices and the components of the magnetic resonance devices are basically less important because the inhomogeneities are estimable and deterministic. The inhomogeneities introduced by the tissue of the patient are more important, as explained, for example, in a summarizing article on the occasion of the “New Zealand MR Users' Meeting” in Auckland, November 1996, with the title “Fat Suppression Techniques”, author Greg Brown, cf. http://www.users.on.net/˜vision/papers/fatsup/fatweb.htm.
Human tissue exhibits a relative magnetic permeability that is different from 1. As a result, the discontinuities between air and tissue and interfaces between different types of tissue having different magnetic susceptibility, for example, lead to relevant (local) basic magnetic field distortions. The inhomogeneous distribution of water/air/bone/fat in the human body also leads to a distortion of the basic magnetic field that is different for each patient.
The field distortion is problematic in the case of extremities (e.g., the knee, the elbow, the arms, the legs, the foot; in the hand/finger region), where sudden, geometrically complex susceptibility changes occur between the anatomy and the surrounding space (e.g., the air or a local coil housing). Local coils for wrists and hands are known in the prior art. The local coils for wrists and hands may have a receptacle around which the coil elements of the local coil (e.g., on the surface of the receptacle) are arranged.
A technique known as “shimming” is known for resolving the problems associated with inhomogeneous basic magnetic fields. While a static shimming operation (e.g., using metal shim elements) may not eliminate the inhomogeneities of the basic magnetic field that are specifically caused by a patient, methods in which shim coils that generate correction fields intended to restore the basic field homogeneity in the imaging region are used are also known. For shim coils of this type, it is known to provide the shim coils, for example, on the housing of the magnetic resonance device, consequently remote from the body. In order therefore to compensate for locally strong field gradients, such as occur, for example, in the hand/finger region due to the sudden, geometrically complex susceptibility changes, very high-order shim coils that have a complex geometry and require a large installation space, as well as having very low efficiency are provided. Such shim coils use a high current and produce a high level of waste heat in return for a relatively small generation of correction fields.